The norm today for the multiplexing of multiple modulated radio signals through a common transmission medium is that the signals' carriers and associated sidebands, as a result of the modulation, must stand alone within the frequency spectrum. There can be no interference from other modulated radio signals or the random intermingling of carriers and sidebands from non-descript signals containing sinusoidal components. Should this occur, the condition of the signals is considered to be inextricably intermingled and no selection of the signals for demodulation can be accomplished without suffering a distortion of the signal*.
The present state of the art requires that each of the signals must be separated so that its sidebands are not overlapping and that the transmission medium has, at least, a bandwidth equal to the sum of the bandwidths of the individual signals being sent. This bandwidth requirement has plagued the bandwidth reduction efforts that have been the target of intense efforts by scientists and engineers working in the telecommunications field. The selection of any one of these separated signals, for eventual demodulation, requires the use of a resonant filter that is designed to pass the particular range of frequencies occupied by the signal and suppresses all other signals at frequencies outside this range. The signals at these “other frequencies” cannot exist within the sidebands of the signal that is being demodulated to avoid distortion.
The instant invention actually eliminates the “stand alone” rule of a signal's spectrum in multiplexed systems. The overlapping of adjacent signal sidebands presents no problem for the instant invention in the extrication for the eventual demodulation of radio signals. The need for a resonant filter circuit for signal selection is not required, and it will be shown, that the unique use of the properties of the product detector circuit is all that is required to facilitate the signal selection process.
Amplitude modulation (AM) is the type of information-carrying signal that is widely used today for communications and is the type of signal associated with this invention. The instant invention selects, simultaneously, pairs of orthogonal AM signals which can be treated as two independent AM signals or as related AM signals thereby allowing the properties of the instant invention to apply to angular modulated signals which includes frequency modulation (FM) and phase modulated signals since the pair of AM signals are the resulting orthogonal components of frequency and phase modulated types of signals.
*There is one known multiplexing method that is an exception as regards to signal sideband intermingling to obtain bandwidth reduction and to increase spectral efficiency, and that is known as “Orthogonal Frequency Division Multiplexing” (OFDM). This method applies only to digital signals that must be generated by a digital signal processing technique and are not independent signals that are modulating independent carriers but a computer-generated subset of digital signals related to a digital main signal. These signals are computer generated to insure that the intermingled sidebands of the computer-generated subset of digital signals are mathematically orthogonal. A digital signal processor is required to generate the signals, their carriers and demodulate the signals. The instant invention requires no signal processors and uses independent digital, or non-digital, signals to modulate carriers of any frequency.